Multimodal Electrophysiologic and Cerebral Perfusion Assessment of Loss of Consciousness in Calves During Slaughter: A Narrative Review

1. Introduction

The determination of unconsciousness in calves during slaughter must be grounded in cortical neurobiology rather than inferred solely from observable motor behavior. Conscious perception, particularly the conscious experience of pain, requires preserved thalamocortical connectivity and functional integration across distributed cortical networks, including primary sensory, associative, and limbic regions [1,2,3]. Experimental neurophysiology in mammals consistently demonstrates that loss of evoked cortical responses and collapse of organized electroencephalographic (EEG) activity are more reliable indicators of loss of consciousness than persistence or absence of brainstem reflexes. In cattle, Daly et al. [4] showed that visually and somatosensory evoked cortical responses were immediately abolished following effective captive bolt stunning, whereas behavioral signs alone did not provide equivalent resolution of cortical state. Similarly, studies in calves undergoing exsanguination demonstrated that electrocortical suppression occurred within seconds, yet some reflexive responses could transiently persist beyond EEG-defined unconsciousness [5,6]. These findings underscore that corneal reflexes, gasping, or limb movements may be mediated at subcortical or spinal levels and therefore cannot independently define conscious awareness.

Within the European Union, slaughter is regulated under Regulation (EC) No 1099/2009, with scientific interpretation guided by the European Food Safety Authority (EFSA). EFSA opinions acknowledge that operational monitoring relies on animal-based measures (ABMs) but emphasize their limitations and the importance of combined indicators to infer unconsciousness. Validation studies in veal calves confirm imperfect concordance between ABMs and EEG-defined cortical states [7].

Calves constitute a distinct neurophysiological subgroup compared with mature cattle, given differences in cranial anatomy, cerebral perfusion patterns, and developmental maturation. Skull thickness influences mechanical energy transfer during stunning, while age-related cerebrovascular characteristics may affect the dynamics of ischemic cortical failure. Integrating electrophysiologic endpoints (EEG and evoked potentials) with cerebral perfusion metrics would provide a mechanistically coherent framework for defining loss of consciousness and clarifying modality-specific pathways to cortical failure in calves.

The finality of this research is to establish a physiologically grounded, multimodal framework integrating EEG and cerebral perfusion metrics to define loss of consciousness in calves during slaughter. By reducing epistemic uncertainty and harmonizing cortical endpoints with operational monitoring, it aims to strengthen scientific rigor, regulatory precision, and welfare assurance within the European context.

2. Methods

This research was conducted as a structured narrative review, in accordance with the Scale for the Assessment of Narrative Review Articles (SANRA) [8]. The SANRA framework was selected to ensure methodological transparency, explicit justification of the review question, systematic literature identification, critical appraisal of included studies, and coherent synthesis of heterogeneous evidence. Given the complexity of neurophysiologic endpoints and the diversity of experimental designs in slaughter research, a narrative approach was deemed more appropriate than a quantitative meta-analysis. The primary objective was interpretative integration of electrophysiologic and cerebral perfusion data rather than statistical aggregation of effect sizes.

A structured literature search was performed in PubMed/MEDLINE from database inception through January 2026. Search strings combined controlled vocabulary terms and free-text keywords related to species (“calf,” “veal,” “cattle”), neurophysiologic measurement (“electroencephalography,” “EEG,” “electrocorticography,” “evoked potentials”), cerebral hemodynamics (“cerebral blood flow,” “perfusion,” “oxygenation”), slaughter modalities (“stunning,” “captive bolt,” “electrical stunning,” “exsanguination,” “neck cutting”), and state descriptors (“unconsciousness,” “insensibility”). Boolean operators were used to maximize sensitivity while preserving conceptual specificity. Reference lists of retrieved articles were manually screened to identify additional eligible studies, particularly earlier experimental reports frequently cited in regulatory opinions.

Studies were eligible if they met the following criteria: 1 – peer-reviewed and indexed in PubMed; 2 – conducted in cattle or calves, with calf-specific data preferentially extracted where available; 3 – included direct neurophysiologic measurements, specifically EEG, electrocorticography (ECoG), or evoked potentials, with or without concomitant cerebral perfusion assessment; and 4 – published in English. Studies were excluded if they lacked direct cortical or perfusion endpoints, relied solely on behavioral indicators without physiologic validation, or involved non-bovine species without clear translational relevance to calf neurophysiology.

Given substantial heterogeneity in experimental paradigms, EEG criteria, perfusion methodologies, and outcome definitions, findings were synthesized qualitatively, emphasizing mechanistic interpretation and cross-study coherence rather than pooled quantitative estimates. The results were synthesized in 4 different areas of interest: Neurobiological Foundations; Electrophysiologic Evidence Across Slaughter Modalities; Cerebral Blood Flow and Cortical Viability; Comparative Mechanistic Interpretation.

3. Neurobiological Foundations

Conscious awareness in mammals depends on preserved thalamocortical processing and large-scale cortical network integration [9]. Contemporary neurobiological models emphasize that wakeful experience requires functional connectivity between thalamic relay nuclei and distributed cortical regions, with dynamic synchronization across sensory and associative networks [10]. Disruption of this integration, whether through mechanical trauma, electrical seizure induction, or ischemia, leads to loss of conscious processing [11]. In large animal models relevant to slaughter pathophysiology, cortical function has been assessed using electroencephalography (EEG) and evoked potentials [12,13]. EEG reflects spontaneous population-level cortical activity, while stimulus-locked evoked responses provide more specific evidence of preserved thalamocortical transmission. In cattle, Daly et al. [4] demonstrated that visually and somatosensory evoked cortical responses were abolished rapidly following effective captive bolt stunning, supporting their utility as markers of cortical non-responsiveness.

Similarly, Gregory et al. [6] showed rapid loss of visually evoked cortical responsiveness during exsanguination in calves. Cerebral perfusion provides the metabolic substrate necessary to sustain cortical electrical activity [14]. Experimental calf data indicate that progressive cerebral hypoperfusion during exsanguination leads to electrocortical suppression within seconds [5]. When cerebral blood flow declines below critical thresholds, organized cortical activity collapses, reinforcing the mechanistic linkage between perfusion failure and loss of consciousness [15]. These concepts are summarized in Figure 1.

4. Electrophysiologic Evidence Across Slaughter Modalities

4.1. Penetrating Captive Bolt (PCB)

Experimental neurophysiologic investigations in calves provide direct cortical evidence regarding the efficacy of penetrating captive bolt (PCB) stunning. In a controlled study using implanted EEG electrodes in veal calves, Lambooy et al. [16] demonstrated that correct frontal or occipital bolt placement induced rapid EEG changes characterized by high-amplitude slow activity rapidly transitioning toward suppression, consistent with abrupt loss of cortical function. In contrast, nuchal (neck) placement resulted in delayed electrocortical suppression, with a mean interval of approximately 21 seconds before EEG criteria compatible with unconsciousness were reached; corneal reflexes persisted until cortical suppression occurred. These findings highlight the critical importance of correct shot placement in determining the temporal profile of cortical failure.

Complementing these observations, Daly et al. [4] assessed cortical function in cattle using electrocorticography (ECoG) combined with visually and somatosensory evoked potentials. They reported rapid and irreversible abolition of evoked cortical responses following effective PCB stunning, indicating interruption of thalamocortical transmission after the impact. Because evoked responses represent stimulus-locked cortical processing, their disappearance provides strong evidence of loss of conscious responsiveness. Together, these studies support the conclusion that correctly applied PCB produces rapid cortical failure, while technical inaccuracies may introduce measurable delay and variability in the onset of unconsciousness. This concept is strongly reinforced by more recent data published in an important scientific journal [17].

4.2. Electrical Stunning

Electrical stunning produces a characteristic neurophysiologic sequence consisting of generalized epileptiform activity followed by post-ictal cortical suppression [5,18]. The initial high-amplitude, synchronized seizure activity reflects widespread depolarization across cortical networks, whereas the subsequent suppression phase corresponds to functional disruption of thalamocortical integration. In adult cattle, Devine et al. [19] documented marked EEG amplitude suppression and altered frequency patterns rapidly following electrical stunning, demonstrating electrocortical changes incompatible with conscious processing.

Head-only electrical stunning induces a reversible state [20]; if exsanguination is delayed, cortical activity may recover as cerebral perfusion is maintained. In contrast, head-to-body electrical stunning combines seizure induction with ventricular fibrillation or cardiac arrest, leading to cessation of cerebral perfusion and eliminating the possibility of neurologic recovery [21]. These mechanistic distinctions are central to interpreting EEG endpoints and assessing the temporal reliability of unconsciousness across electrical stunning modalities.

4.3. Non-Penetrating Captive Bolt

Non-penetrating captive bolt (NPCB) stunning relies exclusively on concussive transmission of kinetic energy without cranial penetration, aiming to induce cerebral concussion sufficient to disrupt cortical integration. In calves, Gibson et al. [22] reported electrocortical suppression following NPCB application under controlled conditions. However, subsequent EEG assessments in bovines indicated greater variability and occasional incomplete cortical suppression compared with penetrating captive bolt methods, underscoring the sensitivity of NPCB efficacy to energy delivery and anatomical factors [17].

4.4. Slaughter Without Stunning

Experimental investigations of exsanguination without prior stunning provide direct neurophysiologic insight into the temporal dynamics of cortical failure in calves. Gregory et al. [6] assessed visually evoked cortical responses during slaughter and reported loss of stimulus-locked cortical activity at approximately 17 seconds following bilateral carotid severance, indicating cessation of functional thalamocortical transmission within this interval. Complementing these findings, a different group of research evaluated spontaneous electrocortical activity in calves and observed rapid EEG suppression within seconds in most animals; however, rare prolonged cases were documented, attributed to carotid problems and incomplete exsanguination, thereby illustrating inter-individual variability in cerebral perfusion dynamics [5]. Importantly, validation studies comparing animal-based measures (ABMs) with electrophysiologic endpoints demonstrate imperfect concordance [7]. This study showed that certain behavioral indicators persisted beyond EEG-defined unconsciousness, while others disappeared prior to cortical suppression, underscoring the limitations of ABMs as standalone markers of conscious state. All together, these data highlight the progressive ischemic mechanism of cortical failure during exsanguination and the potential usefulness of electrophysiologic validation in defining unconsciousness.

All the examined literature is synthesized in Table 1. It is the summary of what has been described before.

5. Cerebral Blood Flow and Cortical Viability

Stunning and non-stunning slaughter modalities converge on the endpoint of cortical non-responsiveness, yet they do so through fundamentally distinct neurophysiologic mechanisms. Penetrating captive bolt stunning produces immediate mechanical disruption of cortical networks, abolishing evoked cortical responses at the moment of impact, as demonstrated by Daly et al. [4] using electrocorticography and stimulus-locked potentials in cattle. Electrical stunning induces generalized epileptiform activity followed by post-ictal suppression of cortical activity, with EEG changes incompatible with conscious processing documented in cattle [19]. In contrast, exsanguination without prior stunning leads to rapid progressive cerebral hypoperfusion, with loss of visually evoked cortical responsiveness occurring approximately 17 seconds after carotid severance in calves [6].

The integration of EEG with cerebral blood flow (CBF) assessment , illustrated in Figure 2, enables mechanistic differentiation between concussive suppression (direct mechanical network disruption), post-ictal suppression (seizure-related transient cortical inactivity), and ischemic cortical failure (perfusion-dependent collapse) [26]. While EEG alone reflects cortical electrical status, concurrent perfusion monitoring provides upstream physiological context [27]. Non-invasive technologies such as near-infrared spectroscopy (NIRS), which estimates regional cerebral oxygenation, and Doppler ultrasonography for flow velocity measurement, offer experimentally feasible methods to correlate perfusion decline with electrophysiologic endpoints [28]. Multimodal integration thus enhances interpretive precision and reduces the risk of misclassifying transient suppression states as definitive loss of consciousness [29].

6. Comparative Mechanistic Interpretation

The principal slaughter modalities differ in their primary neurophysiologic mechanisms yet converge on the endpoint of cortical non-responsiveness. Penetrating captive bolt (PCB) stunning produces rapid mechanical disruption of cortical and subcortical structures. When correctly positioned, abolition of evoked cortical responses occurs in a few seconds, after the impact, supporting rapid loss of thalamocortical integration [4]. However, incorrect placement can delay cortical suppression, as demonstrated in calves, underscoring significant placement-dependent variability [16].

Electrical stunning induces generalized epileptiform activity followed by post-ictal suppression; EEG patterns incompatible with conscious processing have been documented in cattle [19]. Technical parameters, including current density, electrode positioning, and stun-to-stick interval, determine reliability. Non-penetrating captive bolt (NPCB) relies on concussive force; while EEG suppression has been observed, variability is greater than with PCB, reflecting a very high sensitivity to delivered energy and anatomical variables [22].

In contrast, neck cutting without prior stunning induces progressive cerebral hypoperfusion, with loss of visually evoked cortical responsiveness typically within 10–20 s, though inter-individual variability occurs [5,6]. Collectively, the literature supports high probability of rapid unconsciousness under optimized conditions but does not justify claims of absolute certainty across all individuals and operational contexts.

In Figure 3 there is a graphical representation of what is described by the published research. Several aspects are definitely not clear yet, and would deserve further deep investigation.

7. Discussion

7.1. Integrative Interpretation

The convergence of electrophysiologic and cerebral perfusion evidence consistently indicates that cortical non-responsiveness, particularly the abolition of stimulus-locked evoked potentials, constitutes the most defensible operational endpoint for defining unconsciousness in calves [30,31]. Studies using electrocorticography and evoked responses have demonstrated rapid and irreversible cortical disruption following correctly applied penetrating captive bolt stunning [4], while ischemia-driven loss of visually evoked cortical responsiveness during exsanguination occurs within seconds [5,6]. Electrical stunning induces a characteristic seizure-suppression sequence incompatible with conscious processing when technically adequate [19]. Mechanistically, stunning modalities disrupt cortical network integrity directly, whereas exsanguination produces progressive metabolic collapse secondary to cerebral hypoperfusion.

Despite mechanistic coherence, several limitations temper interpretive certainty. Experimental cohorts are often small, limiting robust estimation of tail-risk events. EEG endpoint criteria vary across studies, reducing comparability. Multicenter EU-scale data reflecting operational heterogeneity remain limited. Moreover, simultaneous EEG-cerebral blood flow (CBF) measurements in calves are sparse, constraining physiologic triangulation of cortical failure.

7.2. Future Research Agenda

Aligned with Horizon Europe priorities for sustainable food systems and digital innovation, future investigations should prioritize multicenter EU-scale EEG-CBF validation in calves, development of field-deployable multimodal EEG-oxygenation headsets, and application of machine-learning classifiers to detect cortical failure in real time. Quantification of 99th percentile time-to-cortical-failure distributions would enhance reliability modeling. Harmonization of cortical endpoint definitions across Member States would further strengthen regulatory precision and reduce residual epistemic uncertainty in welfare assessment.

Limitations: This structured narrative review is characterized by several methodological and evidentiary constraints that warrant careful consideration. First, the body of primary literature addressing electrophysiologic determination of unconsciousness in calves remains limited in both scale and breadth. Many of the seminal investigations were conducted in relatively small experimental cohorts under controlled laboratory conditions, which may not fully capture the operational variability encountered in commercial slaughter environments. As a consequence, the capacity to estimate low-frequency or tail-risk events, particularly delayed cortical failure, is inherently restricted.

Second, substantial heterogeneity exists in EEG acquisition and interpretation methodologies. Variations in electrode configuration, recording parameters, signal processing approaches, and criteria for defining cortical suppression or isoelectricity complicate direct cross-study comparison. Moreover, the inconsistent application of evoked potential paradigms limits the harmonization of cortical endpoints across investigations and precludes meaningful quantitative synthesis.

Third, the simultaneous assessment of cerebral blood flow or cerebral oxygenation in conjunction with EEG remains uncommon in bovine research. Thus, mechanistic conclusions linking perfusion dynamics to cortical electrical failure are supported largely by indirect inference rather than robust multimodal datasets.

Finally, as a narrative synthesis, even when structured according to SANRA principles, this review does not meet the methodological rigor of a systematic review or meta-analysis and may be susceptible to publication bias and selective reporting. These limitations, together with all the published data, underscore the necessity for coordinated, multicenter, multimodal investigations aimed at elucidating the precise relationship between cortical electrophysiology and cerebral perfusion in calves.

8. Conclusions

Electroencephalography and stimulus-locked evoked potentials represent the most robust cortical indicators of unconsciousness currently available in calves, as they directly assess thalamocortical responsiveness. Measurement of cerebral perfusion offers complementary physiological validation by defining the metabolic substrate necessary for sustained cortical activity. The integration of electrophysiologic and perfusion metrics therefore constitutes the most scientifically rigorous approach to enhancing welfare monitoring and minimizing residual uncertainty in the assessment of unconsciousness during slaughter.